Concentration-dependent short-range order in the relaxor ferroelectric (1-x)Pb(Sc,Ta)O₃-xPbTiO₃

Using the x-ray diffraction and x-ray absorption fine structure techniques, we have probed Ti, Ta, and Sc local environments in the solid solution system (1-x)Pb(Sc,Ta)O₃-xPbTiO₃. This system is known to display a variety of ferroelectric behaviors ranging from variable order-disorder, to relaxor, to a mixed phase region, and then finally to normal ferroelectric, as the value of x is increased. We find, in agreement with neutron diffraction studies, no detectable displacements of Ta or Sc atoms from their oxygen cage centers in any of these systems. Surprisingly, we find that the Ti atom is displaced along (111) from its inversion symmetry center for x=0.05. However, this average local Ti displacement gradually changes from (111) to (001) as x increases, whereas the global crystal structure abruptly changes from rhombohedral to tetragonal at x=0.45. Our experimental results and theoretical modeling of others together suggest that this system consists of mixed regions, some characterized by a (111) Ti displacement and others characterized by a (001) Ti displacement. The displacement averaged over all regions becomes more weighted toward (001) as x increases. Another significant result is that all our samples (with x ranging from 0 to 0.5) have a high degree of local ordering of the B sites with alternate occupation of Ta and Sc atoms.